2006-07-20 04:00:00 PDT Pasadena -- It was a clear, cold summer afternoon on the Chryse Plain of Mars 30 years ago today, and well before a humid dawn here in California, when a three-footed spacecraft named Viking 1 settled gingerly onto a rock-strewn patch of rust-red Martian sand.

The solar system had welcomed its first interplanetary visitors from Earth along with a Soviet mission to Venus in December 1975, triumphant moments that marked the start of mankind's efforts to probe its neighbor planet for signs of life and set the sights for every Martian mission to follow.

Mars had seen spacecraft fly by in the past; its surface had been photographed and mapped in crude detail from Martian orbit. Even its two small moons, Phobos and Deimos, had been studied from a distance.

But now Mars was host to Earth's first scientific experiments directly on the surface and, a scant few weeks later, a second craft, Viking 2, would follow to land on a dangerously rugged Martian plain misnamed Utopia.

Those two historic spacecraft, separated by 9,600 miles of Martian surface, were to operate for only three months, but they continued gathering data and transmitting it home for six years, and the orbiters that carried them to Mars sent data home, too.

They were followed in 1997 by Pathfinder, an equally historic mission carrying the world's first Mars rover, a tiny six-wheel robot named Sojourner that poked among the Martian rocks with high-tech instruments to analyze their chemistry and photograph their surfaces up close for the first time ever.

And then, in January 2004, came the now famed Spirit and Opportunity, two far more sophisticated Mars Exploration Rovers programmed to last only three months. Today, those two are continuing to trudge across the Martian landscape, confounding and delighting their earthbound guardians -- and there appears to be no end in sight.

Spirit and Opportunity are discovering ever stronger evidence in the rocks and hills of Mars that water, the quintessential ingredient of life, was abundant there millions of years ago -- and may exist just below the surface even now.

What Sojourner, Spirit and Opportunity have accomplished would not have come about had the Vikings never reached Mars safely.

"What a fantastic mission those Vikings accomplished," said Gentry Lee, the space engineer who led the planning team at NASA's Jet Propulsion Laboratory during the Vikings' years of operation on the Martian surface.

"We couldn't imagine what that surface would really be like before they landed," Lee recalled. "We didn't know the rocks on Mars, we didn't know the atmosphere, and while the two landers were still flying around the planet aboard their orbiters, we were still frantically scanning the crude maps we had, trying to pick out safe landing sites -- or what we imagined might be safe.

"We were terrified that one Viking -- or both -- might land on an unexpected rock, tip over and die. It took temerity, and finally when we saw each Viking touch down safely and watched each one go to work, well, talk about exhilaration down here."

It was 5:12 a.m. on July 20, 1976, at Mission Control when Viking 1 landed, ending an 11-month flight and breaking the tension of a final all-night watch by scientists, engineers and scores of reporters who gathered to describe the unfolding of history.

And when the first close-up image of Mars came down across 200 million miles of space only seconds after the landing, it was a sight for wonder.

The picture, displayed on computer and televisions screens, was only black and white and arrived from Viking's primitive transmitter line by line. First came a glimpse of rocks, then a lander footpad the size of a dinner plate, with more rocks in the background and, finally, the lander's own black shadow looking for all the world like some medieval swordsman ready for battle.

It was an awesome vision.

Only a few hours later came the color images, all red rocks and red sand and a sky that at first appeared clear blue but later -- after the images were corrected -- showed pink from the fine Martian dust motes in the atmosphere.

Beyond the Mars landings, Lee, now the chief engineer at the Jet Propulsion Laboratory assigned to assure the integrity of all planetary missions, sees the Vikings as pioneers -- in the design of their aeroshells that must shield all planetary descents from the blazing heat of a planet's upper atmosphere; in the development of parachutes that must deploy safely in slightly lower, much thinner atmospheres; and in the thruster engines that must slow final descents with extreme precision.

"We'd tested the bejesus out of everything we could before those Vikings flew," Lee said in an interview, "and it paid off."

The two Vikings each carried identical suites of biology experiments designed to search for signs of extant life. A long robotic arm on each lander scooped up thimbles full of sand from the surface and deposited one sample in each of three tightly sealed miniature laboratories containing water and nutrients the scientists called "chicken soup" -- nutrients that any microbe might love to consume.

A "gas exchange" experiment was conceived and run by the late Vance Oyama and Harold P. Klein of NASA's Ames Research Center in Mountain View. Klein was Viking's biology chief and is still revered as the virtual founder of the science of astrobiology. If life existed in those thimbles of Martian soil, the organisms might change the composition of gases in the lab environment.

The "pyrolytic release" experiment was headed by the late Norman Horowitz of Caltech, a famed expert on the biochemical evolution of life. In this experiment, the Martian soil was heated by an artificial sun to 1,200 degrees Fahrenheit in a chamber full of radioactive carbon. The idea was to see if any long-dormant microbes would come to life, metabolize and release the labeled carbon compounds.

Finally, the "labeled release" experiment, developed by chemist Gilbert Levin, who today heads a drug technology firm called Spherix in Beltsville, Md., sought to detect living microbes that would be breathing out radioactively labeled carbon dioxide in the Martian soil sample. Levin still stubbornly claims they did. "I think there are organisms there now," Levin insists in papers and presentations at the Society for Optical Engineering.

A separate instrument aboard the Vikings, the gas chromatograph-mass spectrometer, was the final umpire for all the experiments, and its verdict was clear: no carbon dioxide, no organics, only some exotic chemistry on Mars. No life anywhere -- at least not now, only complex chemistry, still poorly understood.

The Vikings and their two orbiters were a billion-dollar mission. Now, the hulks of the Viking spacecraft are monuments on Chryse and Utopia.

Matthew Golombek, chief scientist at the Jet Propulsion lab for the Pathfinder/Sojourner mission, was a geology Ph.D. fresh out of the University of Massachusetts when he joined the Viking science team soon after the landing.

"Viking's life experiments asked audacious questions, and their nul results are still crucial evidence that will guide other missions far into the future," said Golombek, who more recently led the team that selected landing sites for the Spirit and Opportunity rovers exploring the Martian surface and has been analyzing their scientific results.

All the evidence from the Mars missions so far, Golombek said, points to one conclusion:

"The early environment of Mars was habitable. An overwhelming amount of data leaves no question there was liquid water on the surface. But was Mars warm as well as wet? That's what we don't know yet. And did life start there when it started here on Earth? We still need to find out."

Steven Squyres, the chief scientist for the two stalwart Mars rovers still roving today, was a Cornell undergraduate, barely 20 years old when the Vikings landed so long ago.

"Boy, did I follow them closely," he said.

"I'll always look at Viking as the standard against which to judge all other planetary missions. It was the largest leap in technology anyone had ever tried, and just about everything worked.